The present invention is in the field of digital radiography. The invention more specifically relates to a method and an apparatus permitting evaluation of processing performed on the digital signal representation of a radiographic image in a most convenient way.
In the field of digital radiography a wide variety of image acquisition techniques have been developed such as computerised tomography, nuclear magnetic resonance, ultrasound, detection of a radiation image by means of a CCD sensor or a video camera etc.
In still another technique a radiation image, for example an image of x-rays transmitted by an object, is stored in a screen comprising a photostimulable phosphor such as one of the phosphors described in European patent publication 503 702 published on Sep. 16, 1992 and U.S. Ser. No. 07/842,603. The technique for reading out the stored radiation image consists of scanning the screen with stimulating radiation, such as laser light of the appropriate wavelength, detecting the light emitted upon stimulation and converting the emitted light into an electric representation for example by means of a photomultiplier.
The signal is then digitized, processed and subsequently used to control the hard copy recording in image reproducing system such as a laser recorder. For diagnostic purposes the reproduction is viewed and analysed on an lightbox.
In an alternative image acquisition system a digital representation of a radiographic image can be obtained by scanning a radiographic film carrying a radiographic image with light, detecting the image-wise modulated light (reflected or transmitted) and converting the detected light into a digital signal representation. Likewise the digital signal can be processed and can subsequently be used for controlling hard copy recording or display of the processed image.
Previous to image reproduction, the image signal can be subjected to image processing algorithms serving various kinds of purposes and involving the selection and setting of several kinds of processing parameters depending on the specific type of examination.
The different kinds of processing methods that can be applied will be described in extenso furtheron in the application. In summary these processing methods may comprise processing for the purpose of image analysis such as a method of recognising the borders of an irradiation field in case of shielded irradiation and image enhancing processing methods such as spatial frequency processing, noise reduction, gradation correction defining the relation between signal values and corresponding density values etc.
After processing the image signal is commonly sent to an output recorder for reproduction of the radiographic image on film.
The image quality or the outlook of the reproduction does not always match with the expected quality or outlook for various causes which cannot always be assessed easily due to the wide variety of actions that were performed on the image signal prior to reproduction.
When such problems occur, a common practice among radiologists using a digital radiography system of the above-named kind is to collect unsatisfactory hard-copies and to discuss anomalies with the service-technician on his inspection tour.
However, it is sometimes difficult or even impossible to recall certain processing conditions for evaluation or to reconstruct the performed signal processing.
In U.S. Pat. No. 5,004,917 a radiation image reading apparatus has been described wherein reading conditions and/or image processing conditions are established on the basis of the radiation image information which has been read from a photostimulable phosphor plate. Whether the reading and/or image processing conditions fall outside of a predetermined range or not is determined. If the reading conditions and/or the image processing conditions fall outside a predetermined range, the radiation image information and data used to establish the reading conditions and/or the image processing conditions is held in storage.
When the system suffers a fault when it is operated on trial or in actual use it is automatically put into HALT mode, then the user attempts to switch on the apparatus to recover the fault or he calls the service man. If the fault cannot be located, the service man connects a supervising unit. This unit then determines whether established read-out and/or processing parameters fall within a given range, and if negative, the supervising unit stores the reading conditions and/or processing conditions e.g. on a magnetic tape for analysis in the laboratory, so that any fault can be located and repaired.
In this system images for which the processing and/or read-out conditions fall within the predetermined range are not stored, so that in case an anomaly appears when evaluating the hard copy reproduction of such an image there is no possibility of retrieval of the image data foreseen.
It is an object of the present invention to provide a system for reading, processing and reproducing a radiation image, said system being designed so as to provide easy evaluation of the performed processing.
A further object of the invention is to provide a system of the above-named kind wherein the performed processing can be evaluated without the need of examining a hard copy of the processed image.
Further objects will become apparent from the description hereinafter.
The objects of the invention are achieved by a method of evaluating image processing performed on a digital representation of a radiation image comprising the steps of
acquiring a digital representation of a radiation image,
producing for said radiation image an image representation of a reduced number of pixels,
storing in a storage device data identifying an image, said reduced image representation, parameters used in course of said processing and intermediate processing results.
In one embodiment a digital image representation is acquired by reading-out a radiation image that has been stored in a photostimulable phosphor screen. The read-out method comprises the steps of scanning said screen with stimulating irradiation, detecting the light emitted upon stimulation and converting the emitted light into a digital signal representation. This signal representation can then be subjected to image processing.
In an alternative embodiment of the present invention the digital signal representation is obtained by scanning a radiographic film carrying a radiation image with light, detecting image-wise modulated light and converting said detected light into a digital signal representation.
The image-wise modulation of the scanning light can be obtained through either reflection or transmission by the radiographic film. The digital signal is then processed and, in accordance with the present invention, for each read-out image an image representation of a reduced number of pixels is produced, and data identifying an image, said reduced image representation, parameters used in course of said processing and intermediate processing results are stored in a storage device for each read-out image.
Alternative systems for acquiring a digital image representation can be envisioned.
In a preferred embodiment the storage device in which the data pertaining to the radiation images are stored has a limited storage capacity and is sequentially organized so that data pertaining to N successively read-out images can be stored and that when an N+1-th image becomes available, the data pertaining to this N+1-th image are stored at the expense of the first stored image data.
The present invention provides that in case an evaluation of the processing of an image is required, the data pertaining to said image can be retrieved and transmitted to a suitable output device for evaluation. Suitable output devices are a service computer, a display monitor, a hard copy recorder etc.
The method of the present invention is advantageous for the following reasons.
It enables evaluation of the performed processing on the basis of stored digital information, there is no need for collecting hard copies of images.
In this method no preliminary selection criterion is imposed on the data that are stored. In other words, specific data are stored for every read-out image and no essential data pertaining to an image are lost or not stored (of course within the limitation as to the available storage capacity).
For every read-out picture a reduced version is stored and is kept in storage device until the full storage capacity is used. However, the storage capacity needed to perform the method of the present invention is limited since only reduced versions of the image representations are stored.
Among the data that are stored in a storage device are identification data.
In one embodiment of the present invention wherein acquisition of a digital image representation is performed by reading-out a photostimulable phosphor screen, said screen is conveyed in a cassette provided with an electrically erasable programmable read only memory.
In an identification station the patient""s identification data as well as an identifier referring to a preset of processing parameters are written into the EEPROM.
Then, after exposure to x-rays the cassette is put into a read-out apparatus where the screen is taken out of the cassette and scanned with stimulating radiation. Radiation emitted upon stimulation is then directed towards a photomultiplier or the like for conversion of the radiation into an electric signal representation.
Also in the read-out apparatus the data stored in the EEPROM on the cassette are read out. In the processing unit of the read-out device the parameters corresponding with said identifier are retrieved from a parameter table. The indentification data and the processing parameters or at least part thereof are stored for each read-out image in a storage device.
Among the data that are stored for each image are besides the identification data and the processing parameters also intermediate processing results such as the histogram of the original image, the histogram of the image after processing, a noise characterising parameter, etc.
An enumeration of the alternative processing parameters and intermediate processing results that can be stored for each read-out image are given hereinbelow with reference to the drawings.
The following is one example of a set of parameters and intermediate processing results that can be stored.
When processing a digital image signal, first a diagnostically relevant signal range can be determined by evaluation of the histogram of the image as disclosed in our co-pending European application number 91203212.5 filed Dec. 9, 1991 and U.S. Ser. No. 07/978,786.
Then, a gradation mapping function for converting signal values into density values is determined as described in our co-pending European application number 91203209.1 filed Dec. 12, 1991 and U.S. Ser. No. 07/978,091.
This method comprises the steps of
getting a canonical description of the mapping function (from the processing parameters),
getting the minimum and maximum density values (from the processing parameters),
determining maximum and minimum signal values (according to the method disclosed in the above patent application these values are deduced from two intermediary values S0 and S1 by proper relative alignment of the effective mapping range (i.e. the range within which mapping is defined by the above-mentioned canonical description and outside of which signal values are mapped onto said minimum and maximum density) to the relevant signal range [S0, S1].
determining the actual mapping function.
Finally signal-to-density mapping is applied to the image.
Parameters to be determined here are minimum and maximum density value, the parameters defining the canonical form, the percentage indicating the above relative position etc. Intermediate processing results are for example the image histogram used to define the diagnostically relevant signal range, and the minimum and maximum signal values.
Since the parameters are specific for each application and can be adapted relative to the taste of a radiologist, it might happen that the result in the hard copy does not match with the radiologist""s expectations.
Since the above parameters and intermediate processing results and a reduced image representation are in accordance with the present invention stored in the storage device, it is possible to retrieve them for evaluation and/or re-processing.
According to the method of the present invention, for each image a reduced version is produced being a signal representation of a smaller number of pixels than the originally acquired (e.g. read-out from stimulable phosphor screen) number of pixels.
The reduced image representation can be obtained by subsampling the image representation corresponding with the maximum of the acquired (e.g. read-out) pixels.
An alternative and preferred way of obtaining a reduced image version is deduced from an image processing method described in our copending European application number 91202079.9 filed on Aug. 14, 1991 and U.S. Ser. No. 07/924,905.
According to the processing method described in this application an image is first decomposed into a sequence of detail images at multiple resolution levels and a residual image at a resolution level lower than the minimum of said multiple resolution levels.
Next, the pixel values of said detail images are modified to yield pixel values of a set of modified detail images by applying a specific conversion function.
Next the processed image is computed by application of a reconstruction algorithm to the modified detail images and the residual image.
According to one embodiment of this method a detail image at a given resolution level is obtained by computing an approximation image, being a low-pass filtered image representation that has additionally been subsampled.
A detail image is then obtained as a pixelwise difference of the approximation image at a certain step and the approximation image at a next coarser resolution level that was also obtained as described hereinbefore, both images being brought into register by proper interpolation of the latter image.
In this preferred embodiment, the reduced image representation is then obtained as the signal representing one of the lower resolution approximation images calculated during this preferred embodiment of image enhancing processing.
This embodiment of the invention is advantageous since the additional processing time needed for calculation of the reduced version of an image representation is minimal because a reduced image version is already available in the course of the processing.
In a preferred embodiment the storage device is organized as a sequential storage device in which data pertaining to a limited number of images can be stored and in which once the storage capacity is exceeded, the data pertaining to a first stored image are lost at the benefit of data relating to a new image.
In one embodiment of the method of the present invention a kind of xe2x80x9cfreezingxe2x80x9d-functionality may be provided, so that upon activation of said function, no data stored in the storage device can be overwritten or shifted out. This functionality is advantageous because in case very important exceptional data are read and stored, these data cannot be lost during read-out of further photostimulable phosphor screens.
A such-like functionality can be implemented in different ways, for example it is possible to prevent further filling of the sequentially organized storage device once the freeze function has been activated, so that from that moment on no image data are shifted out of the memory queue.
However, in this implementation it is not possible to add then any further new data to the same queue.
If further storage is required, another kind of memory organisation needs to be implemented. For example it is possible to empty a predetermined number of most recently filled memory locations upon activation of a freeze function and to continue filling the queue starting from the remainder of the stored images. Alternatives may be developed in dependence on the specific needs for certain applications.
In another embodiment a single image that is for example selected by scrolling through the queue of stored images and displaying the data pertaining to each of these images on the display of a user interface, can be xe2x80x9cfreezedxe2x80x9d so that it will not be shifted out of the sequentially organized storage device. The storage and shift out of other images remains unaffected.
The invention further provides an apparatus for performing the method of the present invention.
The apparatus generally comprises means for acquiring a digital signal representation of a radiographic image, means for determining a set of processing parameters and means for processing said electric signal representation on the basis of said processing parameters. The apparatus may additionally comprise means for reproducing the processed image.
In accordance with the present invention the apparatus further comprises
means for deducing from said signal representation a reduced version representing a reduced number of pixels,
means for storing in respect of a predefined number of read-out images data identifying an image, processing parameters, intermediate processing results and a reduced version,
means for identifying among the stored data the data pertaining to a specific image,
means for retrieving said identified data,
means for outputting the retrieved data.
In one embodiment means for acquiring a digital signal representation of a radiographic image comprises means for scanning a photostimulable phosphor screen with stimulating irradiation, means for detecting the light emitted upon stimulation and means for converting the detected light into a signal representation.
In an alterative embodiment means for acquiring a digital representation of an image comprises a light source, means for directing light emitted by said light source onto a radiographic film carrying a radiation image, means for detecting image-wise modulated light, means for converting image-wise modulated light into a digital signal representation.
As already mentioned hereinbefore, the apparatus may additionally comprise means for reproducing a processed image . By the words xe2x80x9creproduction of the processed imagexe2x80x9d in this context is meant hard copy recording as well as display.
Hard-copy recording can for example be performed by means of a laser printer. However, alternative printing techniques such as thermal printing (thermal sublimation, wax transfer, resistive ribbon etc.) may be envisioned.
In a preferred embodiment of the present invention the means for deducing a reduced version of the image signal comprise means for performing a pyramidal image decomposition.
Such processing means are described in great detail in our copending unpublished European patent application number 91202079.9 filed on Aug. 14, 1991 and U.S. Ser. No. 07/924,905.
In the course of the processing described in that application, low resolution approximation images are calculated for example by subjecting the read out image signal to consecutive low pass filtering as described below with reference to the drawings.
Hence inherent to this processing is the availability of images at lower resolution levels, so it is preferred to use and store one of these images instead of calculating a dedicated reduced image version because this would increase unnecessarily the computational effort.
The parameters that are determined and stored in the method of the present invention can generally be any kind of parameters that are used during image processing.
For example, it is possible to store the parameters relating to the window-level settings, to the gradation processing and to the processing of detail images described hereinbefore, to noise suppression processing parameters, to processing relating to the determination of a limited irradiation field in case use has been made of a shielding protector at the time of exposure of the patient to x-rays etc.
It is most convenient to store identification data, the processing parameters, the intermediate processing results and the reduced version of the processed image on the system disc. In this way an additional cost of providing additional storage medium has been avoided.
Examples of storage device organisation have been described hereinbefore when explaining the method steps of the present invention.
In one embodiment the means for identifying data pertaining to a stored image comprise a display unit that can for example be part of the user interface of the read-out apparatus.
Means are then provided to initiate upon activation a xe2x80x9cscrollingxe2x80x9d through the stored identification data and simultaneous display of said data on the display unit. Upon command by the operator the scrolling can be stopped when the identification data of an image which is to be evaluated are displayed. Then, retrieval of the remainder of the stored data can then be initiated and the retrieved data can be applied to an output device.
Such an output device can be a printer or a monitor. Alternatively an external storage device can be connected to the read-out apparatus (for example through the intermediary of a service computer). This embodiment provides that the data pertaining to an image to be evaluated can be loaded into the external storage device so that these data can for example be submitted to a laboratory for evaluation.
Means can also be provided for preventing at least one image from being shifted out of the sequentially organized storage device (freeze function).